1. Field of the Invention
This invention relates to corrosive metals and to means for treating such metals to render them resistant to corrosion.
2. Description of the Prior Art
Metal corrosion is a widespread problem throughout the industrialized world, causing losses amounting to several percent of the gross domestic product of the typical industrialized country. Many types of metals are susceptible to corrosion, with aluminum alloys being prominent examples. The protection of metals against corrosion is generally achieved by applying a coating to the exposed surface of the metal to serve as a physical barrier between the metal and the environment. Organic and inorganic coatings have been used, as well as coatings of metals that are themselves non-corrosive.
Inorganic coatings and certain metal coatings such as electroplated hard chrome generally offer the highest wear resistance. The typical inorganic coatings are chemical conversion coatings, glass linings, enamels and cement. Chemical conversion coatings are produced by intentionally corroding the metal surface in a controlled manner to produce an adherent corrosion product that protects the metal from further corrosion. Examples are anodization, phosphatization, and chromatization. Disclosures of conversion coatings are found in:
Aizawa, K., et al. (Henkel Corporation), U.S. Pat. No. 5,904,785, issued May 18, 1999
Rodzewich, E. A., et al., U.S. Pat. No. 5,801,217, issued Sep. 1, 1998
Zheng, H., et al., U.S. Pat. No. 5,807,430, issued Sep. 15, 1998
Isobe, Y., et al., U.S. Pat. No. 5,5,500,288, issued Mar. 19, 1996
Miller, R. N., et al., U.S. Pat. No. 5,399,210, issued Mar. 21, 1995
Glass linings are used in process industries such as the pharmaceutical industry, breweries, and food plants where there is concern over contamination of the product. Enamel coatings, notably porcelain enamel, are used on appliances and plumbing fixtures. Portland cement coatings have been used to protect steel and cast-iron water pipes.
To be effective in protecting the underlying metal from corrosion, the coating must be both resistant to chemicals and impervious to gases and liquids. At present, enamels are the only inorganic coatings that meet these requirements. The application of enamel coatings requires a firing temperature in excess of 500xc2x0 C., however, which renders enamels unsuitable for certain metals, notably aluminum alloys. Accordingly, the most effective inorganic coatings for aluminum alloys at present are chromate conversion and chromic acid anodization coatings. Unfortunately, the application of these coatings involves the use of hexavalent chromium, a carcinogenic substance. This raises environmental hazards and concerns of worker safety, and for this reason many of these coatings have had to be replaced.
Molecular sieves are porous inorganic materials that have uniformly-sized pores whose diameters are either in the micro-size (less than 2 nanometers) or meso-size (2 to 50 nanometer) range. Among the molecular sieves, the best known are zeolites, which are members of a family of minerals known as tectosilicates and are typically microporous crystalline materials with high internal surface area formed by an open, three-dimensional framework consisting of tetrahedral SiO4 units linked together by shared oxygen atoms. Many naturally occurring zeolites exist and many synthetic zeolites have been prepared and are commercially available. In conventional zeolites, a number of the SiO4 units in the framework are replaced by AlO4 units which bear a negative charge which is neutralized by a cation. The choice of the cation influences the properties and use of the zeolite. Many zeolites are synthesized with a high ratio of SiO4 units to AlO4 units, and some are synthesized with no AlO4 units at all. The three-dimensional lattice structure, or xe2x80x9ctopology,xe2x80x9d differs among the various zeolites, and the pore size and pore volume likewise differ accordingly. Zeolites are most commonly used as catalysts and separation media, in each case in the form of a powder composite in which the zeolite is combined with binders. Zeolites have also been disclosed for use as polycrystalline thin films on porous ceramic and metal substrates for membrane and membrane reactor applications. In all of the uses of zeolites to date, the characteristic or quality of the zeolites that has made them useful has been their uniform microporosity.
An additional class of molecular sieves is that of phosphate-containing molecular sieves. In these non-zeolitic molecular sieves, many, if not all, of the SiO4 groups are replaced by PO4 groups as the major structural components of the lattice structure, and the resulting structures offer further variations in the topology relative to zeolites and a wider range of pore structure. These phosphate-containing materials are often used for the same ion-exchange,catalysis, and absorption functions as the zeolitic molecular sieves.
It has now been discovered that corrosion resistance can be imparted to a metal surface by coating the surface with a substantially nonporous molecular sieve. The nonporous character is achieved by incorporating within the voids of the molecular sieve crystal a filler molecule (which may be referred to as a xe2x80x9cpore-filling memberxe2x80x9d) such as those typically used as structure-directing agents in the manufacture of synthetic zeolites and phosphate-containing molecular sieves. The coating is readily applied without the need for extreme conditions and, once applied, the coating displays excellent cohesion to the metal surface. In addition, both the coating method and the coating itself are ecologically harmless. The coating provides corrosion protection in both acidic and basic environments and in environments where aggressive pitting occurs, such as those in which chloride and cupric ions are present. The coating also withstands both thermal and mechanical stresses.
This invention resides both in molecular sieve-coated metal substrates, regardless of how the coating is applied to the substrate, and also in particular methods for applying a corrosion-resistant coating on metal substrates by crystallizing a molecular sieve directly on the substrate. Even though molecular sieves are typically sought for their porosity, this invention demonstrates the unusual discovery that a non-porous molecular sieve has utility that is derived at least in part from its lack of porosity, both inter-crystal and intra-crystal. Utility has thus been achieved by eliminating what has traditionally been considered one of the most significant and valued qualities of molecular sieves.